The reciprocating engine exhaust system is fundamentally a scavenging system that collects and disposes of the high-temperature noxious gases being discharged by the engine. Its main function is to dispose of the gases with complete safety to the airframe and the occupants of the aircraft.
The exhaust system can perform many useful functions, but its first duty is to provide protection against the potentially destructive action of the exhaust gases. Modern exhaust systems, though comparatively light, adequately resist high temperatures, corrosion, and vibration to provide long, trouble-free operation with minimum maintenance.
There are two general types of exhaust systems in use on reciprocating aircraft engines: the short-stack (open) system and the collector system.
Short-Stack vs. Collector Configurations
The short stack system is generally used on nonsupercharged engines and low-powered engines where the noise level is not too objectionable. The short stack system is relatively simple, and its removal and installation consists essentially of removing and installing the hold-down nuts and clamps. Short stack systems have limited use on most modern aircraft.
The collector system is used on most large nonsupercharged engines and on all turbosupercharged engines and installations on which it would improve nacelle streamlining or provide easier maintenance in the nacelle area. On turbosupercharged engines, the exhaust gases must be collected to drive the supercharger turbine. Such systems have individual exhaust headers that empty into a common collector ring with only one outlet.
From this outlet, the hot exhaust gas is routed via a tailpipe to the turbosupercharger that drives the turbine. Although the collector system raises the back pressure of the exhaust system, the gain in horsepower from turbosupercharging more than offsets the loss in horsepower that results from increased back pressure.
Horizontally Opposed Engine Exhaust Components
Figure 1 shows the typical collector exhaust system components of a horizontally opposed engine in side view.
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| Figure 1. Location of a typical collector exhaust system |
The exhaust system in this installation consists of a down-stack from each cylinder, an exhaust collector tube on each side of the engine, and an exhaust ejector assembly protruding aft and down from each side of the firewall. The down-stacks are connected to the cylinders with high temperature locknuts and secured to the exhaust collector tube by ring clamps. A cabin heater exhaust shroud is installed around each collector tube. [Figure 2]
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| Figure 2. A cabin heater exhaust shroud |
The collector tubes terminate at the exhaust ejector openings at the firewall and are tapered to deliver the exhaust gases at the proper velocity to induce airflow through the exhaust ejectors. The exhaust ejectors consist of a throat-and-duct assembly that utilizes the pumping action of the exhaust gases to induce a flow of cooling air through all parts of the engine compartment (augmenter tube action).
Radial Engine Exhaust Collector Ring System
Figure 3 shows the exhaust collector ring installed on a 14-cylinder radial engine.
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| Figure 3. Elements of an exhaust collector ring installed on a radial engine |
The collector ring is a welded corrosion-resistant steel assembly manufactured in seven sections, with each section collecting the exhaust from two cylinders. The sections are graduated in size. [Figure 4]
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| Figure 4. A radial engine exhaust collector ring is graduated in size from the inboard side to the outboard side |
The small sections are on the inboard side, and the largest sections are on the outboard side at the point where the tailpipe connects to the collector ring. Each section of the collector ring is bolted to a bracket on the blower section of the engine and is partly supported by a sleeve connection between the collector ring ports and the short stack on the engine exhaust ports.
The exhaust tailpipe is joined to the collector ring by a telescoping expansion joint, which allows enough slack for the removal of segments of the collector ring without removing the tailpipe. The exhaust tailpipe is a welded, corrosion-resistant steel assembly consisting of the exhaust tailpipe and, on some aircraft, a muff-type heat exchanger.
Manifold and Augmentor Exhaust Assembly
Some radial engines are equipped with a combination exhaust manifold and augmentor assembly. On a typical 18-cylinder engine, two exhaust assemblies and two augmentor assemblies are used. Each manifold assembly collects exhaust gases from nine cylinders and discharges the gases into the forward end of the augmentor assembly.
The exhaust gases are directed into the augmentor bellmouths. The augmentors are designed to produce a venturi effect to draw an increased airflow over the engine to augment engine cooling.
An augmentor vane is located in each tailpipe. When the vane is fully closed, the cross-sectional area of the tailpipe is reduced by approximately 45 percent. The augmentor vanes are operated by an electrical actuator, and indicators adjacent to the augmentor vane switches in the flight deck show vane positions.
The vanes may be moved toward the "closed" position to decrease the airflow velocity through the augmentor, thereby raising the engine temperature. This system is found primarily on older aircraft equipped with radial engines.